1. Field of the Invention
The present invention relates generally to an exhaust gas recirculation (EGR) control system of a type which comprises an EGR passageway having a restriction formed upstream of the EGR control valve to define a chamber between the restriction and the EGR control valve and particularly to an EGR control system of this type in which the EGR control valve is provided with an extension which is inserted into the restriction to reduce the effective cross sectional area thereof and therefore the recirculated exhaust gas flow and the EGR ratio when the pressure in the chamber is reduced below a predetermined value due to increases in an engine suction vacuum during low load engine operating conditions.
2. Description of the Prior Art
As is well known in the art, an exhaust gas recirculation (EGR) control system serves to reduce the production of nitrogen oxides (NOx) in combustion of an internal combustion engine by controlling the maximum combustion temperature below a certain level by recirculating or feeding into air drawn by the engine a portion of exhaust gas emitted from the engine. Accordingly, it is necessary to control the flow of recirculated engine exhaust gas to an optimum value in accordance with engine operating conditions so that the recirculated exhaust gas flow exerts no bad influence on the operating performance or driveability and the fuel consumption of the engine.
It is usually desirable to maintain at about a predetermined or constant value the EGR ratio, that is, the ratio of the recirculated exhaust gas flow to the flow of air taken into the engine. As an expedient for attaining this purpose, there is proposed an EGR control system of a back pressure proportioning type as shown in FIG. 1 of the accompanying drawings. The conventional EGR control system comprises an EGR passageway 1 formed therein with a restriction 2 for controlling the recirculated exhaust gas flow, an EGR control valve 3 disposed in the EGR passageway 1 downstream of the restriction 2 for controlling the pressure Pe in the EGR passageway 1 between the restriction 2 and the EGR control valve 3, and a diaphragm unit including a flexible diaphragm 4 which is operatively connected to the EGR control valve 3 and has on a side thereof a vacuum working chamber 5 fed with an engine suction vacuum. A pressure regulating valve 6 is provided which controls the suction vacuum into the vacuum working chamber by controlling in accordance with the pressure Pe the flow of atmospheric air admitted for diluting the suction vacuum from the engine. the EGR control valve 3 is feedback controlled by the thus controlled suction vacuum in the vacuum working chamber to maintain the pressure Pe constant during all engine operating conditions. Since the recirculated exhaust gas flow depends on the difference between the pressure Pb in the EGR passageway upstream of the restriction and the pressure Pe and the pressure Pe is maintained constant, the recirculated exhaust gas flow is represented as a function of the pressure Pb which is about proportional to the engine taken air flow. As a result, the recirculated exhaust gas flow is controlled to about a constant ratio to the engine taken air flow.
However, when the EGR ratio is controlled to about a constant value throughout all engine operating conditions in this manner, there is a tendency that the stability of operation and the fuel consumption of the engine are degraded during low load engine operating conditions. Since generally, combustion conditions are apt to be degraded to make the operating performance of the engine unstable owing to decreases in the compression pressure of an engine air-fuel mixture and increases in residual gas and furthermore the production of nitrogen oxides is extremely reduced during low load engine operating conditions, it becomes almost unnecessary to perform the EGR. However, it is undesirable to completely stop the EGR since air pollution is promoted and the smoothness of the operating characteristics is lost.